1. Field of the Invention
This invention relates to allocation of priority of lots of work to processing in a factory and more particularly to ranking of lots to determine their priority.
2. Description of Related Art
U.S. Pat. No. 5,319,544 of Schmerer et al. for xe2x80x9cComputerized Inventory Monitoring and Verification System and Methodxe2x80x9d; U.S. Pat. No. 5,128,861 of Kagami et al. for xe2x80x9cInventory Control Method and Systemxe2x80x9d; and U.S. Pat. No. 5,396,432 of Saka et al. for xe2x80x9cVersatile Production System and Method of Operating Samexe2x80x9d show production systems.
U.S. Pat. No. 5,446,671 of Weaver et al. for xe2x80x9cLook-Ahead Method for Maintaining Optimum Queued Quantities of In-Process Parts at a Manufacturing Bottleneckxe2x80x9d shows a production system.
U.S. Pat. No. 5,546,326 of Tai et al. for xe2x80x9cDynamic Dispatching Rule that Uses Long Term Due Date and Short Term Queue Time to Improve Delivery Performancexe2x80x9d; U.S. Pat. No. 5,612,866 of Weng for xe2x80x9cMethod and System for Dynamic Dispatching in Semiconductor Manufacturing Plantsxe2x80x9d; and U.S. Pat. No. 5,721,686 of Shahraray et al. for xe2x80x9cMethod and Apparatus for Control and Evaluation of Jobs in a Factoryxe2x80x9d show scheduling systems.
Budget_Q . . . Budget Queue Time or BQT
BR . . . Base Rate
Critical Ratio.C/R: Critical Ratio Degree of Customer Dissatisfaction
CtR . . . BT (Budget Time) according to Required Date of lot
CtD . . . BT (Budget Time) by the Due Date of lot
CtqR . . . Budget_Q (BQT) according to Required Date of lot
CtqD . . . Budget_Q (BQT) by Due Date of lot
CtI . . . Step Budget_Q (BQT) for Lot Lm, Ln, etc.
CT . . . Cycle Time
Due_Date . . . Scheduled Due Date from MPS, i.e. date when processing of a lot must be finished for an order placed on MPS.
Due_Date_Now . . . Current Scheduled Due Date
FAB . . . Fabrication plant
FFOT . . . Forecast_FAB_Out_Time
Finished out . . . Completed the manufacturing process
MTD . . . Month To Date
MTD_OUT . . . Month To Date wafers OUT of FAB
MPS . . . Master Production Schedule
OTD . . . On-Time-Delivery
P . . . Temporary priority (P value) according to BQT
RP . . . Remaining Theoretical Recipe Processing Time
RD . . . (Required Date) Date when a lot must be finished out according to start date and average past cycle time data.
SLACK . . . Due_Date_Nowxe2x80x94FFOT
Least SLACK . . . Lot with least amount of SLACK
tp . . . time from current step to finished step
tsp . . . Process time from step s from to p (process): Recipe time (process time) by step by product
xcexa3tsp . . . Remaining theoretical process time equal to sum of tp from current step to finished step.
T . . . Time
WIP . . . Work in Process
X . . . Index for Week when lot must be Delivered i.e. Lot is Required to be out of FAB
The required cycle time is based on PC Lot""s RD definition based on the FIFO principle.
Two of the indices most commonly used for setting lot priorities are least SLACK and the critical ratio (C/R). These two indices are used to dispatch with the aim of meeting or improving OTD. However, the values of SLACK or C/R sometimes cannot reflect the actual urgent status of a lot due to the variances of the uncontrollable portion of the remaining Cycle Time (CT), i.e. the remaining recipe processing time between different products. Thus, only after the processing time is deducted while calculating the remaining time, is it possible to define the urgency or the priority of lots clearly. There are two kinds of dates/times when a lot should have completed processing, which are the Required Date (RD) and the Due_Date. The RD denotes the delivery requirement and the Due_Date indicates the shorter cycle time deviation. In practice, the two values, i.e. (the RD and the Due_Date) can be combined together and integrated into a date for calculating the remaining CT. While the combination of the two dates is a convenience; it can lead an operator to be confused by the two different concepts of delivery time and CT variance when evaluating the priority to be assigned to a given lot.
Problems solved and improvements obtained by this invention include the features as follows:
1. With the index of X, the lots are pushed more correctly for delivery especially for the back end stages.
2. With the index of P, the lots are moved according to the remaining Budget Queue Time. This makes lots finished out with little variance.
3. With the distinguishability between delivery and cycle time variance of lots by two parameters of (X,P), the analysis of delivery and variance of the whole line is made easily. Production planners benefit greatly as they are able to make scheduling decisions resolving conflicts arising upon conflicting customer requirements.
The concept of Budget Queue Time is used to define the priority of lots while distinguishing clearly between the controllable an uncontrollable portions of the remaining production time needed and to make the priority setting further meet the actual status. Two indices X and P are used concurrently to define the priority of a lot. X is the index of the delivery week which indicates the week in which the lot must be out of the fabrication process and P denotes the temporary priority according to the Budget Queue Time, but X is the dominant one of the two indices X and P. Use is made of the concept of remaining Budget Queue Time instead of traditional queue time of current stage for dispatching to reduce the variance of cycle time variance.